tests: Fix splicing in functional tests for nix-cli

This is necessary to fix nix-everything-llvm.
The problem here is that nix-cli is taken from the previous
stage that is built with libstdc++, but this derivation builds
plugins with libc++ and the plugin load fails miserably.

tests/functional/package.nix

@@ -60,7 +60,10 @@ mkMesonDerivation (
     ];
 
     nativeBuildInputs = finalAttrs.passthru.externalNativeBuildInputs ++ [
-      nix-cli
+      # Explicitly splice the hostHost variant to fix LLVM tests. The nix-cli
+      # has to be in PATH, but must come from the host context where it's built
+      # with libc++.
+      (nix-cli.__spliced.hostHost or nix-cli)
     ];
 
     buildInputs = [